You can make it work either way but its easier with a sub or subs (especially subs).
In most rooms you run into a problem with standing waves if the room has regular dimensions. This can cause a loss of bass at the listening chair- or maybe (less common) too much bass.
If you have speakers that go to 20Hz and you place them in the ideal spot for imagining and depth this is a very likely problem. I've run into it many times!
The big problem with subs is getting them to blend but if you make sure the sub is operating only below 80Hz it will be a cinch. If its operating over that frequency you may have to place it in the same vicinity as your main speakers; if it only operates below 80Hz it won't attract attention to itself and you can place it where its audible at the listening chair. The thing is that with one sub that placement might not be very convenient. And you could still have problems with standing waves.
An easier approach is to use multiple subs set up as a Distributed Bass Array (DBA). The idea is that since bass is omnidirectional below 80Hz, you can then feed them all the same signal and place them asymmetrically in the room, thus breaking up standing waves and getting evenly distributed bass. The advantage of this approach is that the main speakers no longer have to go down so deep. As long as they get to 50-60 Hz they harmonics of the bass notes will convince you that the note are coming from in front of you even though the fundamentals are coming from the side or even the rear of the room.
Audiokinesis has worked out an effective sub system called the Swarm which is compact, easy to set up and goes right flat to 20Hz.
This opens up the choice of speakers since there are many smaller highly resolved speakers that can handle 50Hz and above quite well- the only issue might be that the speaker behave well if it gets bass notes below its cutoff.
